The present invention relates to charged particle generating apparatus and, more particularly, to an ion implantation apparatus designed to prevent the source vapor materials from condensing on the cool surface of beam-defining member, e.g. the acceleration plate.
Ion implantation apparatus has been adapted for the microfabrication of large-scale integrated semiconductor circuits. For example, U.S. Pat. No. 4,011,449 issued Mar. 8, 1977 to Wen-Chuang Ko et al. describes an ion implantation apparatus adapted to implant impurities into a semiconductor wafer. In such implantation, ion beam emitted from an ion source is shaped and directed toward a target through a plurality of beam-shaping members arranged in well-known manner. Of them, the beam-shaping member that the beam first encounters upon leaving the heated ion source is called the acceleration plate. Conventionally, the acceleration plate is thick and rigid enough to contain mounting surfaces for attachments to an extractor mechanism and has beam-shaping apertures formed therein. Heretofore, no design consideration has been given to an arrangement and/or structure of the acceleration plate for maintaining it at a predetermined high temperature. We have found that the conventional apparatus, having 2mm thick copper as the acceleration plate, cannot maintain the plate at a temperature above the condensation point of the vapor to be implanted, because of the high thermal mass of the thick plate and low spectral emissivity of copper. If a material to be implanted with this apparatus is, for example, arsenic in the vapor state, it is undesirably deposited on the first cool surface, i.e. the acceleration plate, upon leaving the heated ion source. We have also discovered that the build-up of such deposits, after a period of time, begin to flake and cause arcing between the source and the acceleration plate, thereby significantly impairing the operation of the apparatus. One attempt of eliminating the arcing problem is, on a highly periodic basis, to clean the acceleration plate so as to remove the deposits. This makes the apparatus undesirable not only from a utility standpoint but there is an increased safety hazard incurred due to the frequency of cleaning.